Impact of sea spider parasitism on host clams: relationships between burial patterns and parasite loads, somatic condition and survival of host
A sudden outbreak of the parasitic sea spider Nymphonella tapetis accompanied by mass mortality of the Manila (asari) clam Ruditapes philippinarum was observed in Japan in 2007. It was unknown why dead and dying clams were found on the sand surface. In our laboratory experiment, in which sea spiders were artificially parasitised to host clams, unburied clams were significantly increased, suggesting that sea spider parasitism can change the burial mode of clams. We compared parasite loads, somatic condition and reburial activity of Manila clams among three burial groups (fully buried, partly buried and unburied) collected from the field. Unburied and partly buried clams had 30% lower soft body weight and 60% lower adductor strength as compared to the fully buried clams. Parasite prevalence and the mean infection intensity were greater in unburied and partly buried clams than in fully buried clams. Reburial depth was smaller in unburied and partly buried clams than in fully buried clams under laboratory conditions. Approximately 30% of the unburied and partly buried clams died during 30 days, whereas fully buried clams suffered no mortality. These results indicate that sea spider parasitism can reduce the burial depth and somatic condition of host clams, and eventually cause clam mortality on the sand.